The present invention generally relates to apparatus for inspecting integrated circuit (IC) leads. More particularly, the invention relates to an IC lead inspecting apparatus that comprises: at least a displacement sensor for irradiating inspection light onto both an IC setting table and leads of an IC placed thereon to be inspected, and then detecting reflected light to measure the heights of the leads; and a scanning means for moving the displacement sensor relative to the IC setting table and scanning the IC leads by the inspection light. The present invention relates also to a method for inspecting IC leads through measuring the pitch deviation and the coplanarity error of each lead by the use of such inspecting apparatus.
There is a trend toward increased production and supply of QFP ICs to meet the recent rising demand therefor. Consequently, it has become necessary to expedite inspection of a larger quantity of QFP ICs after completion of manufacture. In addition to inspecting the electric characteristics of the ICs, it is considered important to inspect the presence or absence of any deformation in the leads of each QFP IC.
One deformation is pitch error and this refers to skewing of one or more leads or lateral aberrations thereon. Another deformation is coplanarity deformation and this refers to upward or downward bending of one or more leads, when an IC is viewed in plan view. Coplanarity can also refer to leads surface aberrations such as undue roughness.
Due to a numerical increase of leads resulting from the recent progress with regard to high integration density of QFP ICs, the dimensions of the leads and the pitches thereof (i.e., distances between leads) have become smaller, so that even a slight pitch deviation or coplanarity error of a lead deteriorates the mateability between a QFP IC and a wiring film on a printed circuit board to which the QFP IC is connected. It is therefore necessary to be able to inspect the ICs with extreme precision, and this requisite is not met by visual inspection. Further, a satisfactory inspection result is not achievable by sampling inspection alone. Entire productions need to be inspected, this meaning that a great quantity of ICs must be inspected.
In view of such circumstances, apparatus have been developed for lead deformation inspection which employ an image processing system or an optical displacement sensor. Such apparatus are disclosed in Japanese Patent Laid-open No. Hei 1 (1989)-272126 (JP 1272126), the teachings of which are incorporated herein by reference, and No. Sho 63 (1988)-278345 (JP 63278345), the teachings of which are also incorporated herein by reference.
The image processing system of JP 63278345 requires a prolonged signal processing time and therefore has difficulty in meeting the requirement of inspecting a large quantity of QFP ICs. Furthermore, although the inter-lead pitch deviation can be measured by means of an image pickup camera operated above a QFP IC, the upward or downward coplanarity error of each lead cannot be inspected unless an end face image of the lead is picked up by a camera operated beside the QFP IC. Yet further, in inspecting the coplanarity errors on the four sides of the leads, these steps must be repeated four times. Consequently, the time required for inspecting each QFP IC is rendered extremely long.
In the lead deformation inspecting apparatus with an optical displacement sensor disclosed in JP 1272126, a complicated process of image recognition is not necessary, and inspection can be executed in a real time mode merely by processing an electric signal output from an optical displacement sensor, so that both the coplanarity error and the pitch deviation of each lead can be inspected simultaneously by a single displacement sensor, this consequently expediting the operation. In this respect, this system is considered to be advantageous.
In the lead deformation inspecting apparatus of JP 1272126, the sensor includes a light source for emitting a beam of laser light or the like, and a light receiving element, such as a photosensitive diode (PSD), for sensing the light emitted originally from the source and reflected by the lead (or base surface), wherein an output signal of the light receiving element is processed to measure the pitch deviation (deformation in the planar direction of the lead) and the coplanarity error (deformation in the vertical direction of the lead). The principle of such lead deformation inspection is briefly described below.
First, the pitch deviation (deformation in the planar direction of the lead, defect in a lateral portion thereof, or adhesion of extraneous substance thereto) is inspected by relying on the fact that the light emitted from the light source is reflected at different heights depending on the lead and the IC setting table. Such reflection height difference is detected by the displacement sensor, and the inspection is achieved by measuring the timing of the level change caused in the output signal of the displacement sensor by scanning with the inspection light.
The upward or downward coplanarity error (deformation in the vertical direction of the lead) is inspected by first detecting the difference between the heights of the lead and the surface of the IC setting table by means of the displacement sensor and then recognizing the height of the lead on the basis of such difference.
However, in this IC lead inspecting system, the dynamic range of the output signal of the displacement sensor is narrow. This phenomenon is derived from the fact that, since the lead is as thin as 0.1 mm or less, merely a small difference occurs between the output signal of the displacement sensor obtained at the time of scanning the lead surface and the output signal at the time of scanning any positioned space apart from the lead (i.e., the surface of the IC setting table).
With respect to the pitch deviation of the leads, the inter-lead positional correlationship is determined at the beginning and, if any unallowable abnormality exists in such positional correlationship, the condition is regarded as an abnormal state for the first time. Accordingly, when all of the leads on one side of an IC are deformed in parallel with one another, such condition is not regarded as an abnormal state. Such failure to be able to recognize this defect is disadvantageous.
The problem concerning the narrow dynamic range of the output signal of the displacement sensor ca be solved if the surface portion of the IC setting table is composed of a transparent material. Such a solution is composed of a transparent material. Such a solution is disclosed also in Japanese Patent Laid-open No. Hei 1 (1989)-272126. However, in the disclosed structure where the surface portion of the IC setting table is made of a transparent material, a base point for use as a reference for determining the coplanarity error does not exist. Therefore, coplanarity error must be inspected by comparing the heights of the individual leads with one another in a manner similar to inspection of the lead pitch deviation. Consequently, if all of the leads on one side of an IC have the same coplanarity error, it is impossible to recognize such IC as a defective.